This invention claims priority of a German filed patent application DE-A-198 53 093.5.
The invention refers to a method for exposing a substrate, equipped with an n-layer photoresist system, with a corpuscular radiation, an electrically conductive connection being created between a ground potential and the substrate and/or at least one of the layers S1 through Sn of the photoresist system in order to dissipate electrical charges. The invention further refers to an arrangement for carrying out this method.
Methods and arrangements for patterning substrates, for example masks or wafers, in which the substrate is coated with a photoresist and that photoresist layer is exposed to a corpuscular radiation, for example an electron radiation, in order to impress the predefined pattern upon the substrate, are known. For exposure, the substrates are placed onto the support surface of a stage movable in the X and Y coordinates and retained there while the stage is moved step by step in the X and/or Y direction and thereby brought at successive points in time into predefined exposure positions in which the corpuscular radiation is directed at right angles, corresponding to the Z coordinate, onto the photoresist layer.
The photoresist is made of an electrically nonconductive material that becomes electrostatically charged during irradiation. The substrate can also become electrostatically charged. This side effect, referred to generally as xe2x80x9ccharging,xe2x80x9d can unintentionally result, especially in the case of photoresist layer material thicknesses greater than 1 xcexcm, in an influence on the radiation direction of the corpuscular radiation and thus in exposure errors and pattern defects, thus defeating efforts in the microelectronics industry toward increasingly finer patterns. To remedy this, a variety of methods and arrangements for dissipating electrical charges out of the photoresist layer and/or out of the substrate during exposure have been developed.
JP Patent 60-117720, for example, describes an electron beam exposure method in which the electrical charge is dissipated from a specimen equipped with a nitride or oxide layer by the fact that a needle made of a very hard, initially electrically nonconductive material, such as diamond, sapphire, or the like, is made conductive by the implantation of, for example, boron ions and is then used to penetrate through the layer until contact is made with the specimen. This results in grounding of the specimen via the grounded needle, and thus causes dissipation of the charge that has accumulated in the specimen during exposure. Because of the hardness of the material, the needle has a long service life, although relatively large forces must act on the needle in order to penetrate the nitride or oxide layer.
In this context, the advance movement of the needle is limited by the specimen material, i.e. the specimen constitutes the stop for the needle and prevents it from pushing forward into the specimen material beyond a desired degree. Disadvantageously, this method and the arrangement depicted in this context are suitable only for contacting a specimen concealed beneath a layer, by penetration through that layer. If, however, what is provided as the specimen is a substrate onto which a photoresist system made up of several layers has been applied, and if each of the layers is to be individually contacted and connected to a ground potential by way of a separate needle, this procedure is unsuitable, since the needles are always pushed through the entire layer structure until contact is made with the substrate.
A further JP Patent 3-263814 assumes that it is known, for example in the case of a mask board that is equipped with a chromium layer and a photoresist located above the chromium layer, to penetrate through the photoresist layer with the tip of a contact pin, to create an electrically conductive connection to the chromium layer lying therebeneath, and thereby to dissipate the undesired electrons out of the chromium layer through the pin to a ground potential. According to the patent, this method is improved in that the contact pin, upon penetration through the photoresist layer and while contact is made with the chromium layer lying therebeneath, is caused to rotate about its longitudinal axis in order to achieve reliable contact and at the same time to increase the service life of the tip of the contact pin, since the pin can now be of rounded configuration. Leaving aside the increased equipment complexity involved in a rotational drive for the pin, here again the disadvantage exists that contacting of each individual one of a plurality of layers of a photoresist system cannot be performed in this fashion.
JP Patent 2-125416 describes an arrangement for creating an electrical contact between a cassette (ground potential) and a chromium layer that is located on a mask baseplate beneath a photoresist. In this, outside the area that is reserved on the mask of the patterning, a pin is pushed by way of a leaf spring onto the surface of the (electrically nonconductive) photoresist layer. The tip of an electrode that is connected to a high-voltage source is then pushed through the photoresist, in the vicinity of said pin, as far as the chromium layer, and then a voltage of a few hundred to ten thousand volts is applied to the electrode; the photoresist thereby experiences an insulation breakdown as a result of which the pin resting on the photoresist is conductively connected to the chromium layer, and dissipation of charges occurs to the cassette via the pin and the leaf spring.
This technical solution is also not suitable, nor is it provided, for contacting each individual one of a plurality of layers of a photoresist system.
Proceeding from this existing art, it is the object of the invention to develop a method of the kind described above in such a way that in the case of a substrate equipped with a multi-layer photoresist system, before exposure begins each individual one of the layers S1 through Sn of the photoresist system, and if necessary also the substrate itself, is brought into electrically conductive connection with a ground potential.
According to the present invention, the object is achieved in that in a process step before exposure begins, the substrate and/or the layers S1 through Sn are brought into electrically conductive connection with the ground potential by way of a quantity of m contact tips K1 through Km, by the fact that the coated substrate and the contact tips K1 through Km are moved relative to one another until the electrically conductive connection between the ground potential and the substrate and/or each individual layer S1 through Sn is created by way of at least one of the contact tips K1 through Km in each case.
This uncomplicated process step advantageously ensures that there is created, between the ground potential and each individual one of the layers S1 through Sn and the substrate, an ohmic contact by way of which the electrical charges occurring during exposure are effectively dissipated. Any undesired influence on the radiation direction caused by electrical charges is thus prevented, and an essential prerequisite for accurate exposure and for a further refinement in patterns is thus created. The disadvantages of the existing art described above are thus eliminated.
According to the invention, what is achieved in a single process step is that, for example, a contact tip K1 is advanced until contact is made with the layer S1, a contact tip K2 is advanced through the layer S1 until contact is made with the layer S2, a contact tip K3 is advanced through the layers S1 and S2 until contact is made with the layer S3, and so forth, and lastly a contact tip Km is advanced through the layers S1 through Sn until contact is made with the substrate. The electrical charges from the layer S1 are dissipated to the ground potential via the contact tip K1, the electrical charges from the layer S2 via the contact tip K2, from the layer S3 via the contact tip K3, and from the substrate via the contact tip Km.
This procedure at the same time has another advantageous effect, namely that the contact tips K2 through Km dissipate not only the charges from the layers S1 through Sm to which they have been advanced, but also those from the layers S1 through Smxe2x88x92that they have penetrated. For example, the contact tip K2 dissipates not only the electrical charges from the layer S2, but also, because of the ohmic contact created during penetration, those from the layer S1. The same is analogously true of the contact tip K3 with respect to the layers S1 and S2,, and so forth.
In a variant embodiment of the invention, provision can be made for only the layers S1 through Sn of the photoresist system to be connected to ground potential, in the manner described, each via a contact tip K1 through Kn. In this case the number of contact tips is equal to the number of layers.
As an alternative to this, however, it is conceivable for the number of contact tips to be greater than the number of layers. It is then possible to allocate to the individual layers S1 through Sn and/or to the substrate not just one but a plurality of contact tips, and thus to increase the reliability with which electrical charge is dissipated from defined layers or from the substrate. In this context, contact to selected layers and to the substrate can be made with the aid of a plurality of contact tips that are arranged at different positions on the periphery of the wafer or mask. This is advisable in particular if the layers S1 through Sn are segmented, and the individual segments are not electrically interconnected. For example, one contact tip can be assigned to each segment of such a layer.
It is moreover possible in this manner to allocate to each individual layer S1 through Sn and to the substrate, depending on the specific conductivity of the layer material, a number of contact tips which reliably guarantees rapid dissipation of resulting charges from all regions of that layer.
According to the present invention, the manner in which the contact tips are advanced relative to the coated substrate is such that the material of the particular layer to be penetrated by a contact tip is either displaced or removed.
Displacement of the material upon penetration of a layer is attained by the fact that the contact tip, acted upon by a predefined advance force, is guided on a straight motion path, preferably in the Z direction, through that layer. Because of the wedge effect upon penetration of the contact tip into the particular layer and upon further insertion, the layer material is displaced sideways in the X, Y direction.
As an alternative to this, however, a particularly preferred variant embodiment of the invention provides for the advance movement of the contact tips to take place substantially in the Z direction, but for the advance movement to have superimposed on it a component in the direction of coordinate X and/or Y. This laterally directed motion component creates a xe2x80x9cscratch effectxe2x80x9d as the contact tips penetrate, as a result of which the layer materials are removed by the contact tips in locally limited fashion, and intimate contact is thus made with the exposed layer.
The invention furthermore refers to an apparatus for patterning a substrate that is equipped with a plurality of layers S1 through Sn forming a photoresist system, in which a corpuscular radiation is directed onto the photoresist system for the purpose of exposure, and means are provided for dissipating to a ground potential the electrical charges that form during exposure in the photoresist system.
According to the present invention, in an apparatus of this kind at least a number of contact tips corresponding to the number n of layers S1 through Sn is provided, of which one contact tip is assigned to each of the layers S1 through Sn and each of the contact tips is connected to the ground potential via an electrical conductor. In addition, the coated substrate and the contact tips are arranged movably relative to one another in the direction of the corpuscular radiation.
In a preferred variant embodiment, the coated substrate is placed on a support plane that is movable in the direction of the corpuscular radiation as far as a position Z1, while each of the contact tips is mechanically joined to a frame-mounted holding fixture via a separate spring element. Each contact tip is thus movable, in response to a force allocated to it and predefined by the spring element, relative to the other contact tips and also relative to the frame-mounted holding fixture.
This individual mounting of the contact tips on separate spring elements as defined by the present invention makes it possible to assign to each contact, by corresponding design of its spring element, a specific force with which that contact tip acts on the photoresist system when the substrate is shifted into the position Z1.
Provision is also made, according to the present invention, for the individual contact tips to be configured with different geometries in terms of their tip angle xcex1 and tip radius R. It is thus possible, based on known relationships between force and area and in consideration of the thickness and viscosity of the layer materials, to coordinate the spring elements and tip geometries of the contact tips with one another and with the layer materials in such a way that a predefined penetration depth into the photoresist system is achieved when the substrate is shifted into the position Z1.
For example, the tip angle xcex1 and tip radius R of a contact tip K1 and the force F1 allocated to it by the spring element E1 are to be coordinated, with one another and in terms of the thickness and viscosity of a layer S1, in such a way that when the support plane for the substrate is moved to the position Z1, the contact tip K1 cannot penetrate through the layer S1 but rather simply rests on the layer S1, and the electrically conductive connection between the layer S1 and the ground potential connected to the contact tip K1 is created.
Provision is also made, for example, for the geometry of a contact tip K2 and the force F2 to be coordinated, with one another and in terms of the thickness and viscosity of the layer S1, in such a way that when the substrate is moved into the position Z1, the layer S1 is penetrated by the contact tip K2 and the contact tip K2 rests on the layer S2, so that the electrically conductive connection between the layer S2 and the ground potential is created via the contact tip K2. Analogously, the geometry of the contact tip K3 and force F3 are coordinated, with one another and in terms of the thickness and viscosity of the layers S1 and S2, in such a way that when the substrate is moved to the position Z1, the layers S1 and S2 are penetrated by the contact tip K3 and the electrically conductive connection between the layer S3 and the ground potential is created via the contact tip K3.
This applies in the same fashion to the further contact tips; ultimately at least the geometry of the contact tip Kn and force Fn are coordinated, with one another and in terms of the thickness and viscosity of the layers S1 through Snxe2x88x92, in such a way that when the substrate is moved to the position Z1, the layers S1 through Snxe2x88x92are penetrated by the contact tip Kn and the electrically conductive connection between the layer Sn and the ground potential is ensured via the contact tip Kn.
The method and the arrangement according to the present invention are preferably applicable in conjunction with a substrate that is equipped with a photoresist system made up of three layers S1 through S3, the layer S1 comprising a polymer photoresist, preferably of the designation PMMA, the layer S2 comprising silicon nitride, and the layer S3 once again comprising PMMA.
In this context, a very advantageous variant embodiment of the apparatus according to the present invention consists in the fact that four spring elements E1, E2 . . . E4, configured as leaf springs, are present, each of which is joined immovably at one end to the holding fixture while the allocated contact tip K1, K2 . . . K4 is attached at the respective other end. These leaf springs can have imparted to them, as a function of the layer materials and by way of varying configuration of the spring material, spring cross section, and spring length, the properties that, upon movement of the substrate into the position Z1, produce spring forces F1, F2 . . . F4 which ensure that the contact tip K1 reaches its allocated layer S1, the contact tip K2 its allocated layer S2, the contact tip K3 its allocated layer S3, and the contact tip K4 the substrate.
The forces are to be set in a range from 0.1 N to 2 N, and the tip radii of the contact tips are to be embodied in the range from 20 xcexcm to 100 xcexcm, while the thicknesses of the layers are between 0.5 xcexcm and 2 xcexcm.